1. Field of the Invention
The invention is directed to a communication apparatus and more particularly, to a carrier frequency offset calibration method and a carrier frequency offset calibration system.
2. Description of Related Art
A2 stereo system is an analog audio transmission system used in broadcast television systems. Specifically, in order to satisfy users using different languages or to provide a stereo service, during transmission process, many TV programs transmit a format containing two sets of signals (i.e., a primary channel signal and a secondary channel signal) for the users to choose. That is, besides a primary carrier carrying the primary channel audio signal a secondary carrier carrying the secondary channel signal is also provided to achieve a dual sound system. Alternatively, in order to provide the stereo services, audio signals from the left and the right channels are mixed and then respectively transmitted by the primary carrier and the secondary carrier. Generally, frequencies of the primary carrier and the secondary carrier are separated from each other on the spectrum. When receiving a transmission signal, a receiving terminal circuit obtains a primary channel signal and a secondary channel signal according to a known carrier frequency and demodulates the same, respectively, such that a primary channel audio signal and a secondary channel audio signal are obtained, or alternatively, the primary carrier and the secondary carrier are demodulated, remixed to obtain stereo audio signals of the left and the right channels.
However, offsets may occur in frequencies of the carriers of the transmission signal received by the receiving terminal circuit due to discrepancies in the system (i.e., discrepancies between the transmission terminal and the receiving terminal) or affections to the transmission process from the environment, which further results in error occurring when demodulating of the primary channel signal and the secondary channel signal. FIGS. 1A to 1B are schematic diagrams showing a carrier frequency offset. With reference to FIG. 1A and FIG. 1B, carrier frequencies (i.e., center frequencies) of a primary channel signal S0 and a secondary channel signal S1 sent by a transmission terminal circuit are a carrier frequency fc0 and a carrier frequency fc1, respectively. However, after the signals are transmitted to a receiving terminal circuit, as shown in FIG. 1B, a carrier frequency offset may probably occur between the primary carrier and the secondary carrier, such that the carrier frequencies (i.e., the center frequencies) of the primary channel signal S0′ and the secondary channel signal S1′ at the receiving terminal are shifted to fc0′ and fc1′.
In order to avoid the aforementioned situation that the carrier frequency offsets may cause affection to the demodulation of the transmission signal S, a conventional technique utilizes a feedback method for calibrating the carrier frequency offset. FIG. 2 is a schematic diagram illustrating a carrier frequency offset calibration system 100 commonly seen in the conventional art. With reference to FIG. 2 and taking the carrier frequency offset illustrated in FIGS. 1A and 1B for example, the carrier frequency offset calibration system 100 include a standard detector 120, a demodulation apparatus 140 and a demodulation apparatus 160. After the carrier frequency offset calibration system 100 receives a transmission signal S, the standard detector 120 detects which communication protocol that the transmission signal S belongs to and respectively provide a known primary carrier frequency and a known secondary carrier frequency (i.e., standard carrier frequencies, such as the center frequencies fc0 and fc1) belonging to the communication protocol to the demodulation apparatus 140 and the demodulation apparatus 160, and the demodulation apparatus 140 and the demodulation apparatus 160 respectively receive the transmission signal S.
The demodulation apparatus 140 calculates an offset between a primary carrier frequency fc0′ of an actual primary channel signal S0′ and the known primary carrier frequency (i.e., the center frequency fc0) of the primary channel according to the known primary carrier frequency using a mixer 142, a low-pass filter 144 and a carrier offset calculator 148 and serves the offset as a primary carrier offset value f0nos, as shown in FIG. 1B. Then, the primary carrier offset value f0nos is re-fed back to the mixer 142 of the demodulation apparatus 140 to calibrating the carrier frequencies of the transmission signal S and demodulate the transmission signal S using the low-pass filter 144 and a frequency modulation (FM) demodulator 146 to correctly obtain the primary channel signal S0′ (having the center frequency fc0′). The demodulation apparatus 160 utilizes the similar method to calculate an offset between a secondary carrier frequency fc1′ of an actual secondary channel signal S1′ and the known secondary carrier frequency (i.e., the center frequency fc1) of the secondary channel according to the known secondary carrier frequency fc1 using a mixer 162, a low-pass filter 164 and a carrier offset calculator 168 and serve the offset as a secondary carrier offset value f1nos, as shown in FIG. 1B. Thereafter, the secondary carrier offset value f1nos is re-fed back to the mixer 162 of the demodulation apparatus 160 to demodulate the transmission signal S to correctly obtain the secondary channel signal S1′ (having the carrier center frequency fc1′).
In the carrier frequency offset calibration system 100 illustrated in FIG. 2, the demodulation apparatus 140 and the demodulation apparatus 160 separately calibrate and demodulate the carrier frequency offset of the transmission signal S. However, the carrier frequency offset calibration system 100 can not be applied to all carrier frequency offset situations. FIGS. 1C to 1D are schematic diagrams of two types of carrier frequency offsets. With reference to FIGS. 1A and 1C, if, on the spectrum, the secondary channel signal S1′ received by the receiving terminal circuit is more adjacent to the primary channel signal S0 output by the transmission terminal circuit than the primary channel signal S0′ received by the receiving terminal circuit due to the carrier frequency offset, the demodulation apparatus 140 may likely identify the secondary channel signal S1′ as the primary channel signal S0 and demodulate due to carrier frequency calibration, and as a result, the audio signal carried on the primary channel signal S0′ is lost.
In contrary, with reference to FIGS. 1A and 1D, if, on the spectrum, the primary channel signal S0′ received by the receiving terminal circuit is more adjacent to the secondary channel signal S1 output by the transmission terminal circuit than the secondary channel signal S1′ received by the receiving terminal circuit due to the carrier frequency offset, the demodulation apparatus 160 may likely identify the primary channel signal S0′ as the secondary channel signal S1 and demodulate in the demodulation process, and as a result, the audio signal carried on the secondary channel signal S1′ is lost. In view of the foregoing, how to provide a more accurate carrier frequency offset calibration method and system to prevent the audio signals of the primary channel signal S0′ and the secondary channel signal S1′ received by the receiving terminal circuit from being lost due to carrier frequency offset during the demodulation process is still a major subject for persons of the art.